Structural design of conjugated polymers for fluorescence bioimaging in the second near-infrared window

Fluorescence imaging in the second near-infrared (NIR-II) region has become one of the most powerful tools in clinical diagnosis and therapy assessment because of its high spatial resolution, rapid feedback, radiation safety, and low cost. Conjugated polymer nanoparticles (Pdots) based on donor-acceptor (D-A) polymers are some of the most promising fluorescent probes which have many superior characteristics, such as a high fluorescence brightness, good photostability, facile functionalization, and low cytotoxicity. While there has been tremendous progress in developing fluorescent polymers for use in the NIR-II wavelength range, the types of monomer structures used as building blocks for NIR-II fluorophores are still limited compared to those for organic solar cells and organic transistors. This review summarizes the NIR-II fluorescent polymers reported in the past few decades. The donor/acceptor unit structures of the polymers are systematically classified and discussed, which will provide new insights into the logical molecular design of donor/acceptor units for the development of high-brightness NIR-II Pdots.